Our invention is adapted especially to be incorporated in a transmission control system of the kind disclosed in U.S. application Ser. No. 926,627, filed Aug. 10, 1992, entitled "Automatic Transmission Control System", now U.S. Pat. No. 5,305,663, issued Apr. 26, 1994, which is assigned to the assignee of our invention.
The control system of the patent identified above includes three shift valves that are under the control of two solenoid actuators. The shift valves are actuated in a controlled pattern to effect three ratio changes between a low ratio and an intermediate ratio, between an intermediate ratio and a direct-drive ratio, and between a direct-drive ratio and an overdrive ratio. Only two solenoid actuators are needed to control ratio changes between the four driving ratios.
An electronic microprocessor is included in the control system for establishing ratio changes that meet operating demands as determined by sensors that measure driveline variables; i.e., vehicle speed, engine torque, drive range selector position, engine speed and operating temperature, as well as other variables. The microprocessor includes modules capable of performing shift diagnostic strategies for the purpose of detecting when a problem exists in the shift controllers regardless of whether the problem is caused by an electronic failure, a mechanical failure or a hydraulic failure. If a failure is detected, a code in the software for the microprocessor is called out and an appropriate indicator is activated so that a service technician may take the proper repair action to eliminate the problem. The problem signal that is established may be an indicator light (MIL) on the vehicle dash. A special indicator light is illuminated if the functional problem is due to an electronic component failure only rather than due to a failure of the hydraulics or the hardware involved in the ratio change. It is possible, of course, to conduct a pure electrical test of the system if a problem is detected in order to determine whether the electronics is a contributor to the problem.
Our present invention is an extension of known diagnostic strategy since it permits the identification of the electronics as a contributor to a malfunction in the control system without relying solely on an electrical test of the solenoids. It makes possible discrimination of electrical component failures from other failures that might occur. Known diagnostic strategies consider one shift at a time and determine whether that shift occurred properly or not. They do not make it possible to detect whether a shift failure is due solely to a particular solenoid failure.
A control system of the kind disclosed in the patent identified above has a valve system that cooperates with the solenoid valve actuators. The actuators respond to control signals from an electronic microprocessor. A change in state of the solenoid operators results in a shift response as the transmission is conditioned for varying ratios as the vehicle operating conditions require. The solenoid actuators must be operated in sequence as the solenoids change state in order to effect a given ratio change. Thus, each actuator must be carefully calibrated to avoid variations in the responses of the valve system.